The present invention relates to treating thin wafers and, more particularly, to a carrier for wafers to be treated, and wafer treatment apparatus within which the carrier is to be mounted. More specifically, the present invention relates to a carrier for mounting wafers of a semiconductive material within a wafer polisher, and a semiconductor wafer polisher having improved carrier mounting structure.
Semiconductor wafers provide the basic substrate for the formation of integrated circuits. Such wafers are flat discs of a semiconductive material, typically having a thickness less than about 0.5 mm. They most often are of doped silicon and are produced, for example, by first growing a doped, elongated single crystal of the silicon and then slicing the same into water form. One face of each wafer is highly polished and made flat to close tolerances on apparatus designed specifically for such purposes. It is this polished and flat face to which other materials are applied to form desired circuitry.
It should be apparent that the degree to which the wafer face is polished and made flat is quite critical to the formation of reliable semiconductor junctions. For example, before a wafer can be used with today's technology in the manufacture of many large scale integrated circuits, its surface finish must not deviate from absolute flatness by more than a few tenths of a mil.
Most semiconductor wafer polishing apparatuses now used include a carrier to which unpolished wafers are adhered with the faces of the wafers to be polished exposed to a polishing pad. The polishing pad is then brought into pressure engagement with the wafers and both the polishing pad and the carrier are rotated at differential velocities to cause relative lateral motion between the polishing pad and the wafer faces. A colodial silica slurry is provided at the polishing pad-wafer surface interface to aid in the polishing operation.
The carriers which mount the wafers within presently available polishers are generally thin flat plates. Such plates are mounted to the polishing apparatus by being attached to a flat surface of a massive metal backing plate. The backing plate is made massive in an effort to prevent distortion of the same, such as might be caused by thermal changes and the application of pressure to it, and most often has fluid passages distributed throughout its bulk for the passage of a cooling liquid, such as water.
Wafer carrier designs and the polishing machine mounting structures therefor as described, do not enable wafers to be polished predictably to presently required tolerances. One problem is caused by the fact that any deviation in the flatness of the wafer carrier surface will be "telegraphed" through wafers being polished and result in corresponding deviations in the finished wafer face. In present carrier designs, carrier surface irregularities can be caused by many different factors. For example, as mentioned above, most carriers are now attached to a flat surface of a massive backing plate of the polishing machine. It is impractical, however, to achieve absolute flatness in either the backing plate surface or the mating surface of the carrier plate. Thus, the support the backing plate provides the carrier plate is not distributed uniformly over the carrier plate area. This non-uniformity will result in corresponding variations in the flatness of the carrier plate when it is under polishing pressure and, consequently, asperities in the finished surfaces of the polished wafers.
Most carriers are mounted on the polishing machine so that they will tilt around their axes of rotation to assure that their faces to which the wafers are adhered will mate with the flat polishing pad. However, this construction causes its own problems. That is, during relative rotation of the carriers and the polishing pads, tangential forces will be developed at the carrier face-polishing pad interface. These tangential forces will tend to cause unwanted tilting of the carrier and consequent variations in the thickness of the wafers being polished.
Significant heat is generated at the wafer surface during the polishing operation, which heat is conducted to the carrier and its backing plate. Experience has shown that presently designed carriers suffer enough thermal distortion to make achievement of today's flatness tolerances unreliable, in spite of the backing plate being cooled.
The above problems associated with present polishing machine designs are placing a limitation on further advancement in the art of producing microelectronic circuitry. It will therefore be appreciated that advances in the polishing machine field are a must.